1. Field of the Invention
The present invention is in the field of capacitance diaphragm gauges which measure pressure based on the deflection of a diaphragm.
2. Description of the Related Art
Absolute capacitance diaphragm gauges (CDGs) measure pressure by sensing the capacitance change associated with deflection of a diaphragm whereby one side of the diaphragm (“the Px side”) is exposed to the pressure to be measured (Px) and the other side of the diaphragm is exposed to a sealed reference vacuum cavity in which an ultrahigh vacuum (e.g., less than 10−9 Torr) has been created prior to the sealing of the reference cavity.
The CDG measures capacitance between a diaphragm and one or more fixed electrodes housed in the reference vacuum cavity. When the pressure on the Px side of the diaphragm is higher than the pressure in the reference vacuum cavity, the diaphragm deflects in the direction of the fixed electrode (or electrodes), which increases the measured capacitance. As the pressure on the Px side of the diaphragm decreases, the pressure differential across the diaphragm diminishes and the diaphragm moves away from the fixed electrode (or electrodes) in the reference vacuum cavity, which reduces the measured capacitance.
As the pressure on the Px side of the diaphragm approaches the pressure in the reference vacuum cavity, the pressure differential across the diaphragm becomes sufficiently small as to be considered as the “zero point” for the CDG. This fixed zero point is established during the calibration of the CDG and is used as a reference in subsequent pressure measurements.
CDGs are commonly used to the measure pressure in vacuum chambers in which thin or thick films of material are deposited on a substrate. One common example of usage is to measure pressure during the deposition of materials onto the surface of silicon wafers during the fabrication of semiconductor devices. CDGs are quite useful in vacuum deposition processes that utilize multiple gasses because capacitance diaphragm gauges are highly accurate and are able to measure absolute pressure independent of gas composition. Unfortunately, the same characteristics of the CDG that enable the CDG to operate in the pressure regimes in which vacuum deposition is typically carried out also make the CDG extremely sensitive to any form of contamination or coating that finds its way onto the surface of the diaphragm. Compounding the problem is the fact that the diaphragm of a CDG cannot be inspected without removing the CDG from the system. Thus, the user of a conventional CDG has no nondestructive way of looking into the CDG to determine whether the surface of the diaphragm has been contaminated by any type of deposition of material on its surface.
Diaphragm contamination or coating can negatively impact the sensitivity and accuracy of the CDG and can also result in a shift in the zero point of the CDG. Several other commonly encountered phenomena can also impact the sensitivity, the accuracy and the zero point of the CDG. Thus, it has heretofore been impossible for a user to detect the occurrence of diaphragm coating or contamination in real time and in-situ. As a result, users of CDGs have attempted to mitigate the chance of diaphragm contamination or coating by elevating the temperature of the diaphragm. While this technique has long been used and has a positive effect, the technique has not eliminated the occurrence of diaphragm contamination or coating. CDGs are being used more frequently in processes, such as semiconductor wafer processing, that are extremely sensitive to minor inaccuracies in the vacuum measurement. Further, the effects of diaphragm contamination or diaphragm coating on the accuracy and repeatability of the CDG are known to be significant enough to impact process results and process yields.